Journal of Volcanology and Geothermal Research 390 (2020) 106695 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research j ournal homepage: www.elsevier.com/locate/jvolgeores The 6–8 Aug 2019 eruption of ‘Volcano F’ in the Tofua Arc, Tonga a,∗ a a a Philipp A. Brandl , Florian Schmid , Nico Augustin , Ingo Grevemeyer , b a a c a Richard J. Arculus , Colin W. Devey , Sven Petersen , Margaret Stewart , Heidrun Kopp , a,c Mark D. Hannington a GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany b Research School of Earth Sciences, The Australian National University, 142 Mills Road, Acton ACT 2601, Australia c Department of Earth and Environmental Sciences, University of Ottawa, 25 Templeton Street, Ottawa, Ontario K1N 6N5, Canada a r t a b i s c l e i n f o t r a c t Article history: In August 2019 a large raft of pumice appeared in the territorial waters of Tonga. As in many other cases, Received 13 September 2019 this pumice raft was the only surface expression of a major submarine volcanic eruption. Discolored water Received in revised form 23 October 2019 and reconstruction of the drift path of the pumice raft using satellite imagery points towards ‘Volcano F’ Accepted 28 October 2019 in the Tofua Arc NW of the island of Vava’u as the most likely volcanic source. Here we present imagery Available online 9 November 2019 from ESA’s Sentinel-2 satellite that captured the start of the submarine eruption on 6 August 2019 and the waning of the eruption on 8 August, followed by observations of the drifting pumice raft until 14 Keywords: August. This start time is consistent with T-phase records at the seismic stations on Niue Island and Tonga Rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at Tofua Arc 2 or at least near Volcano F. On 8 August, a >136.7 km large raft of pumice appears at the sea surface. Submarine eruption 6 3 6 3 Pumice raft The modelled minimum raft volume is 8.2–41.0*10 m , which is equivalent to 2.5–12.3*10 m dense Island arc rock. The eruption thus corresponds to a volcanic explosivity index (VEI) 2–3 eruption in the submarine environment. Prior to the volcanic eruption, a series of earthquakes close to Volcano F was recorded. The series started on 5 August with a Mb 4.7 event, followed by at least six shallow earthquakes (Mb >3.9) on 6 August. In December 2018 and January 2019, we surveyed the seafloor around Volcano F with multibeam sonar. Combining our data with pre-existing information, we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting. We show that Volcano F represents a major arc volcanic complex that is situated in an extensional setting. The basal diameter of the volcanic apron is >50 km with a large central, 8.7 x 6 km caldera with a floor at ∼700 m water depth. The top of the post-caldera constructional cone complex had a summit depth of 35 m below sea level in 2004. The volcano shows geochemical differences to the adjacent arc volcanoes on Fonualei and Late islands. The volcano’s pristine volcanic morphology and two documented eruptions (2001 and 2019) indicate a highly active volcanic system that warrants further scientific attention. © 2019 Elsevier B.V. All rights reserved. 1. Introduction enhance magmatic differentiation and drive arc rocks to composi- tions generally much more viscous and silicic than at mid-ocean The annual magma budget of the Earth is dominated by melt ridges. Thus, arc volcanoes erupt in various styles ranging from generated at mid-ocean ridges. However, continental and island effusive to catastrophically explosive. Volcanic eruptions and/or arc magmatism represent the second largest contribution to terres- sector collapses of composite arc volcanoes represent a major haz- trial magmatism (cf. Crisp, 1984). Melting at these magmatic arcs is ard but any comprehensive risk assessment is complicated by the driven by the subduction of lithosphere and the devolatilization of fact that many magmatic arcs are intra-oceanic with arc volcanoes the slab at depth. The released fluids trigger melting in the overlying often submerged. This also applies to the >2,500 km-long Tofua- convecting mantle wedge and the resulting ascending arc magmas Kermadec Arc in the Southwest Pacific, where only a few volcanoes are volatile-rich. Volatile load and a thick crust at magmatic arcs rise above sea-level (Fig. 1A). Marine surveys in the late 1980s´ and 1990s´ provided evidence for the high magmatic productivity of the Tofua-Kermadec Arc (Smith and Price, 2006). ∗ Since 1774, a total of 77 volcanic eruptions have been recorded Corresponding author. from the Tongan section of the Tofua-Kermadec Arc alone (Global E-mail address: [email protected] (P.A. Brandl). https://doi.org/10.1016/j.jvolgeores.2019.106695 0377-0273/© 2019 Elsevier B.V. All rights reserved. 2 P.A. Brandl, F. Schmid, N. Augustin et al. / Journal of Volcanology and Geothermal Research 390 (2020) 106695 Fig. 1. A) Overview map of the Tofua Arc. Arc volcanoes are shown by yellow triangles, spreading centers by red lines and the Tonga subduction zone by the black line with triangles. B) Sentinel-2 satellite image of the pumice raft close to the maritime border between Fiji and Tonga on 21 August 2019 (image contains modified Copernicus Sentinel data 2019). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Volcanism Program, 2013). However, direct observations of histor- 2. The August 2019 submarine eruption in Tonga ical submarine volcanic activity remain scarce except for frequent but small eruptions at the Monowai volcanic center (e.g., Chadwick On 15 August 2019, sailing boats encountered a large pumice et al., 2008; Wright et al., 2008; Watts et al., 2012; Metz et al., 2016, raft about 75 km west of Late Island, an active island arc volcano 2019) and direct observations of the 2009 boninite eruption at West in the Tofua Arc of Tonga (Fig. 2). Sulfur smell was present and Mata by submersible (Resing et al., 2011) and subsequent periodic the floating layer of pumice reached up to 30 cm in thickness with remapping (Chadwick, 2019). Besides direct visual observations, individual pieces up to 80 cm in diameter (see detailed report by seismoacoustic T-phases allow volcanic eruptions to be detected Sail Surf ROAM: https://www.facebook.com/sailsurfroam/posts/ over distances of several thousands of kilometers because this sig- 3045949342087617:0; Global Volcanism Program, 2019). How- nal is coupled into the SOFAR channel (cf. Metz et al., 2016). In ever, based on satellite imagery from NASA (https://worldview. many cases, however, pumice rafts floating at the sea surface are earthdata.nasa.gov; accessed on 29 August 2019), the pumice raft the only evidence for recent submarine, silicic eruptions; Contin- first appeared on 8 August in a rear-arc location between the vol- uous Earth observation from space allow to track these rafts and canic islands of Fonualei and Late (Fig. 2). Daily satellite observation trace them back to their initial volcanic source. Previously, there and good visibility (i.e. little cloud cover) allow the pumice raft to be were ten major submarine eruptions recorded from the Tofua- tracked over the corresponding time period, demonstrating that the Kermadec Arc that produced large pumice rafts (see compilation pumice raft encountered on 15 August is the same that was spotted of Bryan et al., 2012 plus the 2012 eruption of Havre: Jutzeler et al., on satellite imagery from 8 August (Fig. 2). A large plume of discol- 2014). These include the 1967 and 1979 eruptions of Metis Shoal ored water is also visible on the images from 8 August and stretches (Bryan et al., 2012), the 1984 and 2006 eruptions of Home Reef from a then-unnamed submarine volcano located in the arc front (Vaughan et al., 2007; Mantas et al., 2011; Bryan et al., 2012), between Fonualei and Late islands and an unnamed submarine vol- the 2001 eruption (Bryan et al., 2004) of the unnamed volcano cano in the rear-arc (Fig. 2). The submarine arc volcano is listed as #243091 (Global Volcanism Program, 2013; listed as #0403-091 #243091 in the database of the Global Volcanism Program (2013) in Bryan et al., 2004), and the 2012 eruption of Havre (Jutzeler and was the source of a previous pumice-raft producing eruption et al., 2014; Carey et al., 2018). In August 2019, reports of a large on 27–28 September 2001 (Bryan et al., 2004; Global Volcanism pumice raft observed in the territorial waters of Tonga and drifting Program, 2019). This volcano was first mapped during RV South- westwards (Fig. 1B) triggered our interest because we surveyed the ern Surveyor cruise SS2004/11 (Northern Tonga Vents Expedition area of the suspected volcanic source during RV Sonne cruise SO- ‘NoToVE’) in 2004 and is listed as ‘Volcano F’ in the correspond- 267 (Archimedes I) in December 2018 to January 2019 (Hannington ing cruise summary (Arculus and SS2004/11 shipboard scientists, et al., 2019). In this manuscript we summarize existing information 2004). In this manuscript we will use Volcano F as the name of the and present new constraints on the August 2019 submarine erup- volcano. The volcano to the west of Volcano F that is in the center of tion in the Tofua Arc of Tonga integrating data from remote sensing, the pumice raft on 8 August (Fig.